One common type of AC to DC converter is the root-mean-square (RMS) converter. The function of an RMS converter is to accept a fluctuating input signal, such as a sinusoidal input signal, and convert the signal to a DC level proportional to the RMS value of the input signal. The accuracy of the conversion is determined by the type of RMS converter, the tolerance of the chosen components, the dynamic range of amplifiers utilized in the converter, the frequency response of the converter, etc. One type of RMS converter is the log-antilog RMS converter, which falls in the general category of computing type RMS converters. One form of log-antilog type RMS converter is the 09A AC/DC Converter (RMS) sold by the John Fluke Mfg. Co., Inc., Mountlake Terrace, Washington, the assignee of the present application, as an accessory for its 8,500 Series of Digital Multimeters.
One of the problems with log-antilog type RMS converters is the different properties they exhibit depending upon whether a positive or a negative signal is applied. More specifically, in the past, it has been assumed that the AC/DC transfer characteristic of a log-antilog type RMS converter has been linear. Linearity has been assumed for both the positive and negative signal paths of such converters. Moreover, crest factor corrections have not been applied. However, these assumptions are not absolutely true. Rather, the transfer characteristic of log-antilog type RMS converters is not precisely linear. Moreover, crest factor variations (which cause gain errors) exist. As a result, the output signal of such RMS converters has not been a true representation of the RMS value of the input signals. While the inaccuracy resulting from the foregoing assumptions may be acceptable in some environments, it is unacceptable in environments where highly precise conversions are required. For example, in an instrumentation environment it is frequently necessary to precisely RMS convert a AC signal into a DC voltage, which is then measured by a precision DC voltage measuring instrument to provide an accurate indication of the RMS value of the AC signal. Depending upon the required accuracy of measurement, a log-antilog type RMS converter may or may not be useful in such an environment. Obviously, it would be desirable to enhance or improve the accuracy of conversion of such RMS converters so they become more useful in instrumentation and other environments where precision RMS conversion is required.
Therefore, it is an object of this invention to provide a method and apparatus for enhancing the output of RMS converters.
It is another object of this invention to provide a method and apparatus for enhancing the output of log-antilog type RMS converters.
It is a further object of this invention to provide a method and apparatus for enhancing the output of log-antilog type RMS converters by correcting for nonlinearities in the AC/DC transfer characteristic of the RMS converter.
It is a still further object of this invention to provide a method and apparatus for enhancing the output of log-antilog type RMS converters by correcting for nonlinearities in the AC/DC transfer characteristic of the RMS converter and correcting for errors due to crest factor variations.